Synthesizing Credit Card Transactions

Synthesizing Credit Card Transactions

Erik R. Altman
IBM T.J. Watson Research Center
1101 Kitchawan Road
Yorktown Heights, NY 10598
ealtman@us.ibm.com
Abstract

As noted by Turing Laureates Geoffrey Hinton and Yan LeCun [11], two elements have been essential to AI’s recent boom: (1) deep neural nets and the theory and practice behind them; and (2) cloud computing with its abundant labeled data and large computing resources.

Abundant labeled data is available for key domains such as images, speech, natural language processing, and recommendation engines. However, there are many other domains where such data is not available, or access to it is highly restricted for privacy reasons, as with health and financial data. Even when abundant data is available, it is often not labeled. Doing such labeling is labor-intensive and non-scalable.

To get around these data problems there have been many proposals to generate synthetic data [25, 21, 20, 17, 19, 2]. However, to the best of our knowledge, key domains still lack labeled data or have at most toy data; or the synthetic data must have access to real data from which it can mimic new data. This paper outlines work to generate realistic synthetic data without those restrictions and for an important domain: credit card transactions – including both normal and fraudulent transactions.

At first glance it may appear simple to generate such transactions – just formalize a few items of the nature, “Sally sold slacks to Sue on Sunday.” However, there are many patterns and correlations in real purchases. And there are millions of merchants and innumerable locations. And those merchants offer a wide variety of goods. Determining who shops where and when becomes daunting. Challenging also is the question of how much people pay. Inserting fraudulent transactions in the mix provides a final challenge.

Addressing these many challenges and generating good data benefits from a mixture of technical approaches and domain knowledge. Those domains of knowledge include mechanics of credit card processing as well as a broad set of consumer domains, from electronics to clothing to hair styling to home improvement and many more. We also find that creation of a virtual world depicting people’s commercial lives facilitates generation of high-quality, realistic data. This paper outlines some of our key techniques and provides evidence that the data generated is indeed realistic.

Although beyond the scope of this paper, our synthetic credit-card data also facilitates development and training of models to predict fraud. Those models coupled with the synthetic dataset also provide foundations for designing acceleration hardware, just as GPUs, TPUs [4, 13] and other devices have been used for domains such as image classification, object detection, natural lanaguage processing, etc.

1 Introduction

As detailed next in Section 2, we use a variety of techniques to synthesize credit card data and have implemented them in a 40,000-line code base. A key element of our approach is an individual consumer so our data generation starts by creating models of individuals. We then create a population of individuals, with aggregate characteristics mimic’ing their distribution in the real population. Our initial efforts are US-focused, so we broad characteristics are representative of the United States, e.g. in age, occupation, income, credit scores, geographic distribution, etc.

A first requirement of such statistics is that they match the mean and standard deviation of the real population. Generally census and other data sources allow this requirement to be met in a straightforward, albeit sometimes time-consuming manner. Once we have means and standard deviations for statistics of interest, we select specific values for individuals by stochastic sampling, generally from a Gaussian distribution.

A fine point, but a key point here is the difference between the population distribution and an individual’s distribution. For example, the population (and different subgroups among it) have distributions for spending on certain categories, e.g. spending on restaurant meals. Once the mean and standard deviation of an individual’s restaurant spending is selected from the population values, the individual spends according to their personal distribution – and does not redraw from the population numbers for each purchase. Without this distribution for the individual as opposed to the population, an individual’s spending would seem to fluctuate randomly with high, medium, and low spending in proportion to the population.

However, there is a larger challenge than getting good values for mean and standard deviation. That challenge is obtaining accurate cross-correlations between different metrics for an individual. There are two primary reasons:

  1. Pairwise correlations are not available for every pair of statistics, e.g. haircare spending by US zipcode. Even transitivity does not provide full or precise data, e.g. when the correlation between A and B is X, and between B and C is Y. Some pairs of data have no transitive connection, but even when they do, combining X and Y generally yields a wide range for the correlation between A and C.

  2. Given generated data for two series A and B – each with proper mean and standard deviation – data for A and B must be transformed to create a specified correlation, while still maintaining the original means and standard deviations. Thankfully, there are standard mathematical techniques using singular value decomposition which perform this operation [12]. Unfortunately these techniques are expensive both in memory and number of computations. Thus, we also employ more ad hoc techniques in some cases. For example, foreign travel tends to increase with wealth.

A population of individuals each with their own characteristics is a start. However, actual behavior of those individuals must be instantiated. For example, what does a person buy? When and where do they buy it? How do they buy it, e.g. cash, a particular credit card, etc? We detail those apsects in Section 2.

However, broadly speaking we simulate artificial worlds. People live in particular places, they travel for business and for pleasure, they don’t work most weekends, they buy things, etc. To generate data, we log the interactions people have in our simulated world. Although we have not yet taken broad advantage of the capability, our simulation approach enables generation of heterogeneous data sets that are almost never available in real data. For example we could create unified datasets with credit card transactions, loan applications, travel, callcenter conversations, and medical data. There are connections between all of these activities: people may purchase an item and then call to report a problem with it. They may buy airline tickets and then travel. Medical visits incur expenses that paid by credit or debit card.

As these interplays may suggest, another key component of our simulation system is state machines. For example, is a person in the Travel state or the Home state. The set of activities and purchases while at Home typically are different than in Travel. Similarly, purchases tend to vary between Weekdays and Weekends and between Morning, Afternoon, and Night. Our simulations use these states and the transitions between them to generate more realistic data. This state machine approach couples well with our stochastic selection of values from a distribution described above: An individual’s activities happen in natural sequence. However, the activities are not unrealistically mechanical. For example, chances are astronomically low that we would generate a person who pays the same amount for dinner at the same restaurant at precisely the same minute every Saturday evening. However, a person may typically eat at restaurants on Saturday evenings, with one restaurant being a particular favorite. Furthermore, there may be concept drift [5] – behaviors shifting over time. Aside from the periodic shifts noted above and the Home / Travel distinction, we model such things as retirement and later, extreme age.

Another essential issue in generating data is accuracy and fidelity to real behavior. Our general approach to this problem is to compare synthetic data to easily (and automatically) ascertained properties of real data. For example is synthesized spending on credit cards similar to real spending? Do people spend on the same sorts of things? Are fraud levels similar? Section 4 provides more details and examples attesting to the realism of our approach.

Once these automated checks have been done and the system is properly configured, human experts can also be called upon to look at small samples of synthetic data and check for any issues not detected by the automatic assessment. By the nature of its sampled approach, this human check can be increased or decreased depending on the number of people available and the budget for paying such people. We emphasize that the measures (such as mean spending per transaction or distribution of spending by merchant type) employed in automatic comparisons do not require deep learning or human labeled data.

Synthetic data has other benefits, e.g. it can be generated in different ways so as to address particular challenges, e.g.

  • Improve explainability of results Generate data with a desired set of characteristics, e.g. a narrow range, bimodal values, etc.

  • Avoid bias in results Generate data where two classes have statistically identical behaviors, and then check if the outputs of particular models are also statistically identical.

  • “Natural data” is sparse or unbalanced Generate data that fills in the sparse areas or provides unrealistically high activity in a normally under-represented segment.

Our hardware colleagues have been industrious and successful in generating chips and systems to accelerate training and inference, e.g. with GPUs [4] or more specialized chips such as Google TPUs [13] or other chips [10, 9]. However hardware has been developed and optimized primarily for learning domains in which there is abundant labeled data, e.g. image classfication, object detection, speech recogntion, natural language processing, translation, etc. Learning domains such as credit card fraud that we target with synthetic data do not have such accelerators. Or at least hardware designers did not have the performance of these domains as a primary design consideration – due to the relative lack of data and models. Synthetic data can change this situation and broaden the set of learning domains participating in the virtuous circle of accuracy and performance improvements.

2 Synthesized Credit Card Data: Approach

As noted in the Introduction, we generate synthetic credit card transactions via simulation of a virtual world. 111For brevity we generally say just “credit cards”, but unless otherwise indicated that designation also includes debit cards and prepaid cards. That virtual world has a population of consumers with characteristics such as age, income, and geographic location in proportion to the overall population of the United States. (Eventually we hope to extend our model to have consumers across the world.)

The virtual world also has a population of merchants. Like consumers, merchants embody many real-world characteristics. For example we model total sales amount by merchant category. For credit cards, categories are labeled by a Merchant Category Code or MCC. MCC codes range from 0 - 9999.) We model both large multinational merchants like Apple (MCC=5045) and McDonalds (MCC=5814) as well as local merchants such as dry cleaners (MCC=7210). The model has over 300 multinationals, each with many physical locations and in most cases an online presence. Unlike consumers, merchants are not limited to the U.S, but are distributed around the world. (Consumers based in the U.S. may travel anywhere in the world, and a key component of fraud detection is separating actual consumer travel and consequent purchases from fraudulent activity.) Altogether our model has over 16 million merchant locations at which consumers can shop.

Unfortunately a representation that includes only consumers and merchants makes it challenging to determine specific stores where transactions should occur. Consumers often shop for specific items, e.g. clothing or groceries or a necklace. Those items and most others can be purchased at a broad set of MCCs – from stores like Walmart (MCC=5311) selling a wide range of merchandise to stores selling a narrow range of items like a local jeweler (MCC=5094). Thus, internal to our virtual world we have created a list of almost 100 types of Goods and Services that people purchase, and a mapping from to the set of MCCs at which the goods and services can be purchased. The MCCs then map to merchants and specific merchant locations.

also has other features, e.g. how frequently an item is purchased. (This frequency is actually a distribution ranging from the fraction of people who buy the item multiple times per day to the fraction that buy the only once a decade.) also captures time-of-day tendencies, e.g. that people are more likely to visit a bar in the evening than the morning. Similarly notes the relative proclivity to consume an item on weekdays vs weekends and whether at home, on vacation, or on business travel. Finally each item in has an income distribution – indicating both the likelihood that people buy the item and the amount they spend on it if they do buy it. These characteristics are then translated into specific (and different) preferences for each consumer.

However, just knowing tendencies and preferences does not adequately capture consumer behavior. As noted in the Introduction, our virtual world includes state machines to reflect causal relationships, for example making purchases at merchants relatively close to the consumer’s current location.

3 Synthesized Credit Card Data: Examples

Figure 1: Sample Bio of Consumer, “Leia Butler”
Figure 2: Sample Info: Card of Consumer, “Leia Butler”
Figure 3: Sample: Transactions of Consumer, “Leia Butler”

 

  • Mean and Standard Deviation for Cards per Consumer, Cards per Account, Transactions per Year, FICO Score, Income, Debt as fraction of income, Credit Limit, Balance as fraction of Credit Limit, Years Account Open, Years since last PIN change

  • Mean Annual Weekend Getaways

  • Mean Annual Vacations

  • Mean Vacation Duration

  • Mean Annual Business Trips

  • Mean Business Trip Duration

  • Probability of Foreign Weekend Getaway

  • Probability of Foreign Vacation

  • Probability of Foreign Business Trip

 

Table 1: Selected parameters controlling credit virtual world

To make these notions concrete, Figure 1 shows sample biographical data of one consumer (“Leia Butler”). The bottom of Figure 1 notes that Leia has 3 cards. Figure 2 then shows one of the 3 cards (a debit card) generated for Leia. Finally, Figure 3 shows a sampling of transactions on that card. These transactions are generated using the stochastic sampling techniques and state machines described above, and with the parameters in Table 1 as some of the key inputs.

Synthesizing Fraud

A key purpose of generating synthetic credit card data is to help train models to do a better job of detecting fraud. As such, the virtual world must include not only genuine transactions between consumers and merchants, but also fraudulent transactions. We have implemented two mechanisms for fraud:

The first mechanism creates a population of fraudsters similar to the population of consumers. Each fraudster lives in a particular place, has particular preferences for items purchased, days of the week when purchases are made, etc. Each fraudster is also active for a particular time range – from months to years. This model comports with real observations on fraudster behavior [7]. It also reflects that for most fraudsters, using cards for false purchases is not a hobby but their job. Like other jobs, fraud is carried out on a particular schedule and in particular places. And like other jobs, workers enter and leave the field.

The second mechanism generates fraudulent transactions at random points in time for each consumer. This random mechanism could represent a worst-case future scenario when fraudsters have determined how to randomize their purchases among stolen cards so that there is little apparent pattern to their purchases. This case represents another benefit of synthetic data: the potential to get ahead of the curve of real data, and to determine how well models work in hypothetical what-if scenarios.

With a population of fraudsters as in the first mechanism, we also label the generated transactions with the identity of the fraudster. Using this label for training a model is of course forbidden: real transactions never come with such labels. However, fraudster labels have two benefits:

  • The labels can ease debugging and understanding of models. If a model does particularly well or poorly identifying fraud from a particular fraudster, that info can be used to further tweak the model and improve its accuracy.

  • Our generation of synthetic transactions is independent of the model for detecting fraud. Thus, during data generation it is not known how quickly a model will detect fraudulent transactions and revoke the card to stop the fraud. Once a model detects fraud with from fraudster on card , the model can throw away future transactions on card . This capability is not available when training with real data. If the deployed fraud-detection model detects fraud on at a particular time, there will be no future fraudulent transactions on . Thus, when models are trained on real data, they become dependent on the behavior of previous models.

Synthesizing Other Attributes

Another important element of our virtual credit card world is modeling the chip / non-chip status of credit cards and debit cards. Chips generating unique transaction identifiers were introduced on a large scale in the U.S. in 2014. Compared to the previous magnetic stripe technology, the chip’s unique identifiers make it harder to perpetrate “card-present” fraud. As a result of (1) this chip technology; (2) increasing numbers of online transactions; and (3) increasing thefts of credit card information from large online repositories – online purchases now dominate credit card fraud. Approximately 70% of fraud in the U.S. now happens online [6]. Europe adopted chip cards earlier and saw online fraud increase commensurately sooner.

Our model can generate consumers over an arbitrary period of time. We typically start in the mid 1980s and simulate until the present. We model a number of the changes over this span. For example, online transactions start in the mid 1990s and gradually grow to present levels. As just noted, levels of online fraud also increase significantly in the last few years. Over this long time period, 18-year old consumers and others also emerge and begin using cards for the first time. Others retire and their pattern of purchases change.

Data from such long time periods is unavailable in the small number of real data sets on which work has been published [22, 23, 3] – yielding another benefit of synthetic data. Reports about fraud detection using real data have periods ranging from 2 days to a few months to a one year. However, many purchases typical of fraud occur at long time intervals. For example, foreign trips are separated by years for many people. Expensive purchases such as furniture, jewelry, and high-end electronics also tend to be purchased relatively infrequently. Like travel, these purchases are disproportionately represented in fraud. Thus data spanning long time periods is important to separating real transactions from fraud.

Our synthetic data has all credit and debit cards of a consumer as well as their cash purchases. Real data sets are typically limited to transactions from one card or at most one family of cards (e.g. Visa or Mastercard) and never include cash. As such, synthetic data provides a means for assessing how much accuracy is lost due to unavailability of “full” data sets. A broad set of synthetic data also provides a foundation for transfer learning and augmenting real data (as opposed to totally supplanting it as we do).

4 Results

 

 

Table 2: Summary stats of biographical attributes

To fine-tune data generation and provide fidelity with the real world, we look at many population statistics. Table 2 summarizes across the biographical attributes listed in Figure 1. If the summary stats do not match what is desired, we can adjust the values in Table 1 and others until the population aggregates have the desired values.

 

 

Table 3: Summary stats of credit, debit and prepaid cards

Similarly and as analog to Figure 2, Table 3 provides a summary across the credit, debit, and prepaid cards of individuals. The numbers in Tables 2 and 3 are indeed reflective of the broad U.S. population, e.g. roughly equal numbers of men and women, mean FICO score of 712, mean income, vacation days, etc.

(a) Lifetime Statistics on Transactions
(b) Annual Statistics on Transactions
(c) Annual Statistics on Spending
(d) Statistics on Spending per Transaction
Table 4: Summary stats: transactions and spending

It is also useful to look at summary statistics on transactions at varying granularities. In particular we look at (a) lifetime transactions – over the approximately 35 years of the virtual world in Table 4(a); (b) annual transaction counts in Table 4(b); (c) annual spending in Table 4(c); and (d) per-transaction spending in Table 4(d).

The numbers in these tables reflect actual values, e.g. the amounts of different transaction types in Table 4(d) – for both fraud and non-fraud. Similarly the usage ratio of credit, debit, and prepaid cards in Table 4(a) is accurate, as is the higher rate for online fraud than card-present. We also hope that readers will find the transactions in Figure 3 realistic.

We tabulate many other statistics beyond the data shown in these tables, e.g. statistics per U.S. state, per country, and per MCC, as well as various online statistics. We omit details here for brevity.

Once input parameters are such that summary statistics for the generated data match desired values, we can generate arbitrarily large datasets. We have generated datasets spanning 35 years with 20,000 consumers performing more than 300 million transactions. Represented in CSV format, such a dataset requires over 20 GBytes. Arbitrarily larger datasets are possible as needs dictate and storage resources allow.

5 Related Work

Many previous works assess fraud-detection models [15, 16]. The largest number have been built around a public-domain Kaggle dataset [14] with about 280,000 transactions collected over 2 days. Our transaction count is more than 1000x larger and spans decades, not days. Unlike our data the Kaggle data also obfuscates all features except transaction dollar amount and time. This obfuscation is done via a principal components transformation which creates uncorrelated numerical features. This lack of correlation is unrealistic. Having no underlying intuitive sense of the features also increases the difficulty of building models. Capital One has blogged about their internal work using GANs to generate synthetic transaction data [2]. However their approach requires access to real data, which is then amplified to create new data. Our approach requires no access to real transactions.

Other studies have been done on more realistic (non-public) data sets [22, 23, 3]. However, the time span for these studies ranges from 3-12 months versus 35 years for our data. The maximum number of transactions in these previous studies is around 10 million – less than 1/30-th of the number in our synthetic data, and we can generate datasets that are far larger still. As has been observed in other domains, the quantity of data matters in achieving high model accuracy.

More broadly synthetic data can be viewed as a complement to techniques such as transfer learning [24] and few-shot learning [18, 1] that learn based on a small set of data. However, instead of having models learn from a small amount of data or from results in a related domain, our approach generates data from fundamental principles of how things work and simulations embodying those principles.

6 Conclusions

We have outlined techniques for synthesizing credit card transactions for U.S.-based consumers purchasing in the U.S. and world-wide. We have also provided statistics and transaction snippets indicating that the results are realistic.

Improvements are always possible. As future work we plan to support broader populations than the U.S. We also plan to enhance the state model in our virtual world to provide yet more realism in individual consumer behavior. We also plan to examine how GANs [8] could systemically improve our data.

Further afield, many techniques outlined here can be applied to sythesize other types of data. Bank loan applications have many overlaps as do patient medical records. Medical records have at least as many privacy restrictions as credit card data and can also benefit from a virtual world approach for modeling behavior, disease progression, etc.

Transcripts are available for a large corpus of speech. However, except in a few cases involving large human effort, transcripts do not provide the underlying semantic intent of the words. Synthetic approaches could prove helpful. Automatically interpreting charts and graphs is another challenge where synthetic data may help.

Aside from new domains and improvements in real-world fidelity, we plan to investigate improved technical approaches. For example with thousands of variables, generating cross-correlations between all pairs is computationally expensive. Can we improve it?

7 Acknowledgements

In the course of numerous conversations my colleagues at IBM have provided much useful feedback and insight. In particular I thank Shyam Ramji, Ravi Nair, and Jeetu Raj.

References

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